This invention relates to an injection molding process for synthetic resin and its apparatus, wherein a gate communicating with a cavity is cooled forcibly to solidify the synthetic resin in the gate after each injection molding operation, thereby the gate is closed.
Generally, synthetic resin injection molding machines of this sort are of a sprue runner type and a sprue runnerless type. The precision injection molding is mainly of the former type. However, the injection molding of this type is forced to consume sprue runners as scraps, so that it becomes a wasteful consumption of material and disadvantageous in view of the energy saving policy.
According to the sprue runnerless type injection molding, the exterior and interior of the runner are constantly heated by a heater in order to prevent the synthetic resin in the runner from cooling and solidifying. Therefore, it is avoided to consume the material wastefully. However, the inconvenience of it is that since the synthetic resin in the gate communicating with the cavity is subject to a back pressure by a return movement of a plunger means for the injection molding, so that it may flow back into the runner. Thus, the molding accuracy is bad and the molded articles may be deformed or not uniform in shape. On the other hand, to prevent this back flow, it is required to maintain the inner pressure of the cavity until the fused resin within the cavity is solidified. However, the inconvenience is that the molding time becomes prolonged.
For the purpose of eliminating such inconveniences of the conventional art, the present Applicant proposes novel injection molding methods and apparatus particularly in Japanese Patents 762148, 916450, 902601 (the corresponding U.S. Pat. No. 3,800,027), etc. According to one aspect of the new techniques disclosed in the aforesaid patents, an intermittent heater is disposed in a gate, thereby the synthetic resin in the gate is fused by the intermittent heater and injected in the cavity. Due to stoppage of the heating operation, the fused resin in the gate is radiated and solidified, thereby the fused resin in the cavity is prevented from flowing back into the runner. Thus, a wasteful consumption of the material is avoided and a suitable mold operation is realized.